This invention relates to a method for the surface treatment of a material by exposing the surface to ion bombardment caused by generating electric discharge within a vacuumized airtight container.
The glow discharge surface treatment which is one form of surface treatment performed on a metallic material effects desired cleaning of the surface of the metallic material by disposing the metallic material as a cathode within an airtight container with the surface thereof for treatment opposed to an anode, vacuumizing the container, introducing into the container a gas for glow discharge of the order of 10.sup.-2 Torr, and applying a high-voltage DC current between the opposed electrodes thereby generating glow discharge therein. Thus, the metallic material, before being subjected to a reactive evaporation treatment, has the surface thereof cleaned by applying a DC voltage of -1000 V to -2000 V to the metallic material enclosed with an inert gas such as argon under a vacuum of about 10.sup.-2 Torr thereby generating glow discharge and bombarding the surface of the metallic material with ionized argon species.
This method, while in the interval between the time the sputter cleaning treatment is completed and the time the reactive evaporation treatment is started, necessitates the work of evacuating the gas used for sputter cleaning, introducing a reactive gas for reactive evaporation, and adjusting the pressure of the reactive gas thus introduced. For this method, therefore, the interval between the displacement of the two gases and the adjustment of the gas pressure constitutes lost time. According to this discharge method, the density of the current flowing into the material under treatment is on the order of 0.1 mA/cm.sup.2. For the efficiency of the cleaning to be enhanced, therefore, the current density must be elevated.
Since the mean free path for collision of gaseous atoms and molecules is on the order of less than 1 mm under a pressure of about 10.sup.-2 Torr, the ions directed to bombard the metallic material under treatment loose their energy because of the inelastic collision with gaseous atoms and molecules before they reach the metallic material. The method thus has another disadvantage that the frequency of collison with high-speed ions is lowered. When a steel material under treatment is coated with titanium nitride, for example, the steel material is desired to have its surface hardened by nitriding in advance so that the material may acquire improved adherence and wear resistance. When the glow discharge is effected in nitrogen gas under the aforementioned conditions, the metallic material cannot be given effective nitriding. Under these conditions, the temperature elevation of the material under treatment is small (not more than about 120.degree. C.). When the coating is desired to be carried out at a temperature of more than 200.degree. C., for example, the metallic material under treatment requires a heat source of its own. Owing to these disadvantages, the glow discharge treatment has been in need of improvement.